Apple says,
11.2 Apps utilizing a system other than the In App Purchase API (IAP) to purchase content, functionality, or services in an app will be rejected.
For example my IOS app will be used to make consultations with a web service and will be given free to users by my customer, since the web service is ours and flexible, it can offer different and hunderds of types of consultations (e.g first-aid guide, TV troubleshooting, Internet Connection trouble shooting..etc) and this can all be updated in web service by user. But this does not mean that users can use the app to buy some new guides. It will be offered to the users for free with a specific description of “This app includes this 10 guides” but we are charging the customer for license of using out web service and iPhone is one of the many ways we are offering client to access the web service and get that knowledge.
Is this possible? and what are the Apple restrictions?
Can I sell this app to CompanyX which offers 10 different troubleshooting guides?
Then sell CompnayQ another build with supports 50 different guides?
Apple have made it clear in their terms for the App Store that you must not offer anything for sale – neither products nor services – from inside an app unless it uses their purchasing APIs. This is why e. g. Amazon had to remove the buy-a-book feature from their Kindle app for iOS. You must not even have a link to a website in your app that would take the user to your website for purchase in Safari. AFAIK even a text hint on some screen inside your app telling people to go to your website for purchase of further services might be problematic in the review process.
In my opinion (which is just that, an opinion) you will probably be good with different apps for CustomerX and CustomerY, each offering free access to a specific subset of your web services. You will even be good if existing users of any of the apps can buy access to additional services on your website and then use them in their respective apps, as long as you do not link to that page from your app. You will, of course, still have to implement some kind of user-id system to recognize which users have access to the additional services, and which don’t.
I suggest you take a look at how Amazon does it, because their Kindle app has certainly had a lot of scrutiny from the reviewers. Follow their lead and you should be good.